2484
NOTES
in excess, the total pressure of the optical cell was measured with a silicone oil manometer and a cathetometer. After the excess chlorine was account,ed for, the data of the five runs agreed very well throughout the spectrum. For example, the maximum in the visible region was 116.5 (mole-cm.jl.)-l with a variance of 0.9 (mole-cm.11.1-2. Our results are the arithmetic average of the data of the five runs,
Results In Figure 1 our results have been compared to the data of Seery and Britton,2 Binder3 a t 18", and Gibson and R a m ~ p e r g e r . ~A line has been passed through our data and Binder's. Our data are not reported a t
0 SEERY AND BRITTON --GIBSON AND RAMSPERGER 0 NEBEKER AND PINGS
250
300
3%
400 WAVELENGTH.
450
500
550
Me
Figure 1. Uecadic extinction coefficients of iodine monochloride.
wave lengths greater than 576 mp because this is the convergence limit, above which the absorption spectrum is d i ~ c r e t e . ~In the ultraviolet region, our measurements fall on the same curve as those of Binder, with Seery and Britton's results deviating slightly from this curve. However, in the region between 290 and 370 mp, the measurements of Binder and ourselves differ greatly from those of Seery and Britton. Also, since their data in the visible region are slightly displaced toward shorter wave lengths, we suspect that their instrument may not have been accurately calibrated. \Ye calibrated the Cary Model 14 spectrophotometer wave length scale by independent measurements and found the accuracy to be well within the 4-A%. tolerance specification of the instrument. Severtheless, our data agree fairly well with those of Seery and Britt011 in the visible region, differing by 47, a t the maximum. Seery and Britton also noticed that their data did not agree with that of Gibson and R a m ~ p e r g e r . ~ Hotvever, Gibson and Itamsperger may not have reported t,rut. extinction coefficients. ( 5 ) 0. Dnrhy:ihircL, P h y s . Rew., 40, 366 (1932).
The Journal o! Physical Chemistru
Difluoroacetylene'
by Julian Heicklen and Vester Knight Aerospace Corporation, El Segundo, Calijornia (Received February 263 f965)
We wish to report the preparation and purification of difluoroacetylene. The preparation of this compound has not been previously reported except in two patents.2 In the first, Gochenour claimed a method of preparation for all of the perfluorinated alkynes though he presents no evidence that he ever prepared C2F2. In the second patent, difluoronialeic anhydride was thermally decomposed a t 500-1000". The reported products of the reaction were FC=CCOF, C2Fz, C02, and CO, and the polymers of the fluorinated compounds. The presence of C2F2 was inferred by distilling the condensed products of the pyrolysis directly into a mass spectrometer and noting that the peak a t m/e 62 was prominent. While such evidence was never isolated. is highly suggestive, the CzFz We prepared C2F2 from the photolysis of CzF4. Four different experiments were performed. h sample a t 20 mm. pressure was photolyzed in a quartz cell with a Hanovia spiral low-pressure mercury resonance lamp or a Hanovia Type-SH U-shaped medium-pressure niercury lamp. In both cases, runs were made with and without a Corning ?-54 filter. This filter removed radiation below 2200 A. With the low-pressure resonance lamp, the reaction cell remained cold, whereas with the medium-pressure lamp, the cell became hot, probably reaching temperatures between 100 and 200". In all the experiments, c-C3FGwas produced owing to double-bond cleavage followed by addition of CF2 radicals to C2F4.3y4 Additional products were found only in the experiments with the medium-pressure lamp when no filters were used. Under these conditions, the only additional products were CZF2, the polymer of C2F4, arid perhaps in some experiments a trace of some C4 fluorocarbons, even when photolysis was sufficiently long to exhaust the C ~ F Jmonomer. (We did not analyze for CFI, so it might also have been present.) The room-teniperaoture mercury-sensitized photolysis of C2F4 a t 2537 A. does riot break C-F ( I ) This u.ork was supported by the U. S. Alr Force under Contract
AF 04(695)-269. (2) (a) r. I. Gochenour, U. S. Patent 2,546.997 (1951); (b) U'. J.
;\[&Jeton. u. s. patent 2,831.835 (1958). (3, B. Atkinson, J . Chem. SOC.,2684 (1952). (4) J. P. Heicklen, V . Knight, and 8 . A. Greene, J . Chem. Phys., 42, 221 (1965).
COMMUNICATIONS TO THE EDITOR
2485
bonds. 3, * Apparently, the medium-pressure lamp emits radiation below 2200 8. which can cleave the C-F bond. The mechanism that explains the results with 2537-,k. radiation is314
+ hv(2537 K.)+Hg* Hg* + CzF4 +Hg + 2CFz CFz + CzF4 + c - C ~ F ~ Hg
2CF2 +CzF4
(1) (2)
(3)
(4)
When higher-energy radiation is also present, then CzF4
A.)+C2F3 + F
-I- h~(
